The present invention relates to semiconductor laser drive circuits for use in laser printers, optical disc drives, optical networking apparatuses, and so on, which control driving of a semiconductor laser diode serving as a light source.
Due to the recent downsizing and high-speed modulation of drive current, semiconductor laser diodes are widely used as a light source in laser printers and similar devices.
However, the relationship between the drive current of a laser diode (hereinafter referred to as LD drive current) and an optical output (i.e., a laser light), can considerably change depending upon temperature. As a result, an automatic power control (hereinafter referred to as an APC) circuit that adjusts light intensity to be constant is generally required. As a result, various APC circuits, for example, a sample hold (hereinafter referred to as a S/H) type APC circuit, have been proposed.
In such a S/H type APC circuit, an optical output of a laser diode is converted into current (hereinafter referred to as monitor current) and voltage, in turn, by a photo detector during a light intensity adjusting period. To obtain a prescribed light intensity, a LD's drive current is adjusted in such a manner, that a voltage converted from the monitor current which is proportional to the laser light intensity, regardless of temperature, becomes equivalent to a prescribed reference. After the adjustment, the LD's drive current is continuously modulated into a prescribed light intensity having an adjusted value.
Further, when an abnormality is detected in a laser diode or a power supply, the laser diode is forcibly turned off by a resetting operation while discharging electric charge of a S/H capacitance, and thus, decreases a LD's drive current to avoid deterioration of the laser diode, as described in Japanese Patent Registered No. 3302352.However, if the electric charge of the S/H capacitance is discharged, a charging operation should be started from a zero level in the next APC. One attempt to resolve such a problem is to charge the S/H capacitance at a prescribed voltage before the APC, as discussed in Japanese Patent Application Laid Open No. 10-93171.
However, a S/H capacitance is generally charged at a prescribed constant voltage. Whereas, the threshold current of a laser diode tends to vary due to uneven manufacturing processes and temperature in practical use. Further, a LD's drive current generated by a voltage of the S/H capacitance varies in accordance with the precision of current/voltage conversion in an APC circuit. As a result, the laser drive current sometimes largely exceeds the threshold current and excessively drives and damages the laser diode.
To resolve such problems, one attempt is to decrease the above-mentioned prescribed constant voltage to a prescribed level. However, a voltage appearing at the S/H capacitance when the APC is terminated does not reach a target level, resulting in insufficient LD drive current.
Further, with increased speeds in instruments, an adjusting time period starting from when reset (e.g., an LD is forcibly turned off) is cancelled to when the APC control is terminated, tends to be short. Accordingly, if the S/H capacitance can be rapidly charged up to a target voltage appearing at the end of the APC, as close as possible, the adjusting time period can be short. Further, it is desirable to charge the laser diode regardless of the type and temperature while avoiding breakdown.